The present invention relates to a process for increasing the efficiency of heat recovery and improving heat integration with direct product quenching in the selective conversion of oxygenates to olefins.
Light olefins (defined herein as ethylene, propylene, butenes and mixtures thereof) serve as feeds for the production of numerous important chemicals and polymers. Light olefins traditionally are produced by cracking petroleum feeds. Because of the limited supply and escalating cost of petroleum feeds, the cost of producing olefins from petroleum sources has increased steadily. Efforts to develop and improve olefin production technologies, particularly light olefins production technologies, based on alternative feedstocks have increased.
An important type of alternative feedstocks for the production of light olefins are oxygenates, such as alcohols, particularly methanol and ethanol, dimethyl ether, methyl ethyl ether, methyl formate, and dimethyl carbonate. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal wastes, agricultural products, or most organic materials. Because of the wide variety of raw material sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum feedstock source for olefin production.
The conversion of oxygenates to olefins takes place at a relatively high temperature, generally higher than about 250xc2x0 C., preferably higher than about 300xc2x0 C. Because the conversion reaction is exothermic, the effluent typically has a higher temperature than the initial temperature in the reactor. Many methods and/or process schemes have been proposed to manage the heat of reaction generated from the oxygenate conversion reaction inside of the reactor in order to avoid temperature surges and hot spots, and thereby to reduce the rate of catalyst deactivation and reduce the production of undesirable products, such as methane, ethane, carbon monoxide and carbonaceous deposits or coke. It would be very useful to have a process that effectively utilizes the heat of reaction contained in the products exiting the oxygenate conversion reactor, optimizes heat recovery, and reduces overall utility consumption in the conversion of oxygenates to olefins. Such a process is environmentally, economically, and commercially more attractive.
In the conventional systems, the oxygenate conversion reaction is predominantly conducted in the vapor phase using feedstocks and diluents that are usually liquid at ambient conditions. This requires supplying substantial heat to the process to boil the oxygenate feedstock prior to introducing it to the reactor, conventionally supplied by steam heat exchange or furnaces. Loss of energy is incurred in these indirect heat exchange methods, and substantial equipment is required. For steam, boilers must be built in addition to a steam/feed exchanger, and construction of a furnace is more expensive and complicated than a traditional heat exchanger. Methods are needed to improve the energy efficiency of the oxygenate conversion process and reduce the cost of providing vaporized oxygenate feedstock to an oxygenate conversion reactor.
Energy efficiency and cost of providing vaporized oxygenate feed is further complicated if utilization of a diluent is desired. The most commonly noted diluent, water/steam as disclosed in U.S. Pat. No. 5,714,662, requires substantial energy and equipment cost to generate, but has the advantage of being easily able to separate from desired light olefins (especially ethylene and propylene). Other very commonly noted diluents such as inert gases, including nitrogen, helium and even methane (see U.S. Pat. No. 5,744,680) require no energy or equipment to vaporize, but require extensive energy and equipment to separate from the desired light olefin product. Further, use of diluents can allow high total pressures while providing low oxygenate partial pressures, which can be advantageous in reducing compression energy needed in the overall (including olefin separation and recovery) oxygenate conversion process, but this benefit may be outweighed by the energy costs of boiling and separating of the diluent just noted. Proper selection of diluent composition is also needed to improve the energy efficiency in the overall process and reduce the cost of providing vaporized oxygenate feedstock to an oxygenate conversion reactor.
The present invention provides a process for converting an oxygenate to olefins with increased heat recovery and heat integration, said process comprising: heating a feedstock comprising said oxygenate having a first heat content from a first temperature to a second temperature through from one to about three stages having successively higher heat contents; contacting said feedstock at said second temperature with a catalyst comprising a molecular sieve under conditions effective to produce a deactivated catalyst having carbonaceous deposits and a product comprising said olefins, wherein said molecular sieve comprises pores having a pore diameter smaller than about 10 Angstroms and said product has a third temperature which is higher than said second temperature; quenching said product with a medium at an initial temperature and in an amount sufficient for forming a light product fraction and a heavy product fraction wherein said light product fraction comprises light olefins and said heavy product fraction has a final temperature which is higher than said first temperature by at least about 5xc2x0 C.; using said heavy product fraction to provide heat at one or more of said stages to achieve said higher heat contents.
In another preferred embodiment, the process for converting an oxygenate to olefins comprises providing a feedstock comprising the oxygenate, transferring heat from at least a portion of an effluent of an oxygenate conversion reactor to the feedstock to cause at least a portion of the feedstock to vaporize and form a vaporized feedstock, and contacting the vaporized feedstock at a temperature from about 200 to about 750xc2x0 C. and a pressure from 1 kPa to 100 MPa with a catalyst comprising a molecular sieve having a pore diameter smaller than 10 Angstroms, wherein the feedstock has a boiling range of no greater than about 30xc2x0 C., the oxygenate conversion reactor converts at least a portion of the feedstock into the effluent, and the effluent comprises the olefins.
In another preferred embodiment, the process for converting an oxygenate to olefins comprises providing a feedstock comprising the oxygenate and a diluent, transferring heat from at least a portion of an effluent of an oxygenate conversion reactor to the feedstock to cause at least a portion of the feedstock to vaporize and form a vaporized feedstock, and separating the diluent from the effluent, wherein the feedstock has a boiling range of no greater than about 30xc2x0 C., the oxygenate conversion reactor converts at least a portion of the feedstock into the effluent, and the effluent comprises the olefins.
In another preferred embodiment, the process for converting an oxygenate to olefins comprises providing a feedstock comprising the oxygenate and a diluent, transferring heat from at least a portion of an effluent of an oxygenate conversion reactor to the feedstock to cause at least a portion of the feedstock to vaporize and form a vaporized feedstock, contacting the vaporized feedstock at a temperature from about 200 to about 750xc2x0 C. and a pressure from 1 kPa to 100 MPa with a catalyst comprising a molecular sieve having a pore diameter smaller than 10 Angstroms, and separating the diluent from the effluent, wherein the feedstock has a boiling range of no greater than about 30xc2x0 C., the oxygenate conversion reactor converts at least a portion of the feedstock into the effluent, and the effluent comprises the olefins.